1. Field of the Invention
The present invention relates to non-volatile storage devices.
2. Description of the Related Art
Materials having a detectable level of change in state, such as a resistance, are used to form various types of non-volatile semiconductor based memory cells. It has been proposed that such memory cells might be used for binary data storage in memory arrays by assigning a lower resistance state of a memory cells to a first logical state such as logical ‘0,’ and assigning a higher resistance state of the memory cell to a second logical state such as logical ‘1. ’ Other logical data assignments to resistance states may also be used. Some materials can be reset back to a higher resistance state after being set from an initial state to a lower resistance state. These types of materials can be used to form re-writable memory cells. Multiple levels of detectable resistance in materials might be used to form multi-state memory cells which may or may not be re-writable.
One type of memory cell that exhibits switching behavior between at least two resistance states is referred to as ReRAM for “resistive switching RAM”. ReRAM may also be referred to as R-RAM or RRAM. A ReRAM memory cell may include a first electrode, a re-writable switching material (also referred to as a state change element), and a second electrode. The switching material may be metal oxide (MeOx). The ReRAM device may also have a steering device such as a diode electrically in series with the other elements. Switching the memory cell between resistance states may be achieved by applying a voltage across the memory cell. An alternative way of explaining the switching between resistance states is to force a current through the memory cell.
The mechanism by which a ReRAM memory cell switches between resistance states is not completely understood. However, it has been suggested that current paths (in the form of filaments) appear in the MeOx switching material when a sufficient voltage bias (“set voltage”) is applied. The filaments may be broken (reset) by applying a “reset voltage”, resulting in higher resistance. The filaments may be due to oxygen vacancies in the MeOx switching material. These oxygen vacancies may be created, annihilated and moved (from one electrode towards the other) during the set and reset processes. Thus, for some memory cells a “set operation” is used to establish a “set resistance” and a “reset operation” is used to establish a “reset resistance”. The reset resistance may be thousands or even millions of times greater than the set resistance.
During the reset operation a certain current flows through the memory cell (“reset current”). Reducing the current that is needed to reset the memory cell may be beneficial in that it may reduce disturb issues and reduce stress on the memory cell. Reducing stress increases the number of set/reset cycles that are possible. Reducing the required reset current can also reduce power requirements.
Note that the desired properties for the first and second electrode may be different from each other due to the asymmetry of operation of the memory cell. For example, during a set operation oxygen vacancies may be created in the MeOx at the interface between the MeOx and one of the electrodes. For convenience of discussion, the electrodes will be referred to as “top” and “bottom” electrodes. The terms top and bottom are relative to voltages that are applied to the memory cell during operation. For purposes of discussion the electrode that has a positive voltage (relative to the other electrode) applied to it during a set operation will be referred to as a “top electrode”. The other electrode will be referred to as the “bottom electrode.”
Various combinations of materials have been proposed for the MeOx switching material and the electrodes. One possible combination that has been suggested is ZnMnO for the MeOx switching material with platinum (Pt) or some other conductive MeOx such as TiOx as the top electrode and TiN or oxidized TiN as the bottom electrode. Another possibility is a transition MeOx for the switching material with Pt or some other conductive MeOx as the top electrode and TiN as the bottom electrode. Still another possibility for the MeOx switching material is NiO with Pt as the top electrode.
While platinum may be a very effective material for the electrodes, platinum is not a good material for semiconductor fabrication. Therefore, it is desirable to find an alternative to platinum for the electrodes. Moreover, choosing the combination of materials for the MeOx switching material and the electrodes is a difficult task.
Also, the methods of fabricating the memory cell should be compatible with existing semiconductor fabrication techniques. For example, fabrication of other elements might require the use of high temperatures. In one implementation, fabricating a steering element diode may require annealing at temperatures as high as 750 C. Therefore, the materials for the memory cell should be able to withstand high temperatures.